This can be achieved through separate external measuring devices. However, for reasons of cost for manufacture and assembly, and also for better protection against mechanical damages, it is typically more advantageous to dispose the required position sensor for the piston in the interior of the piston accumulator, so it is protected.
This causes a variety of problems.
On the one hand side, such position sensors are often operated electrically or at least analyzed electrically, regard less of their functional principle, so that a supply and return of electric power and/or data in electrical form has to be performed into and out of the interior of the piston accumulator, which as a matter of principle provides a source for leaks, in particular when the sensor is disposed in the pressure loaded portion of the piston accumulator.
Quite frequently, a measuring cable travel distance sensor was used as a sensor, wherein a measuring cable is wound onto a cable drum that is preloaded by a spring in windup direction.
When a measuring cable sensor of this type is attached at a face of a piston accumulator and a loose end of a measuring cable is attached to a piston, a position of the piston in a cylinder can be detected by detecting rotations of a cable drum.
In this context, it is already known from EP 1979716 to connect one or plural encoder magnets with the cable drum torque proof, and to dispose a measuring cable sensor of this type, thus without an electrical supply, as a purely mechanical component completely encapsulated in the interior of the piston accumulator.
The rotations of the one or plural encoder magnets co-rotating with the cable drum are sensed by an angle sensor that is sensitive to magnetic fields from outside of the cylinder through the pressure tight wall of the cylinder and the piston position is determined there from.
Thus, the pressure tightness of the cylinder is maintained.
For the wall of the cylinder, it is stated that it is required that it is made from a non-magnetizable material in the portion of the detection of the encoder magnet. Also, this solution, however, has disadvantages in turn:
When the measuring cable sensor is installed with the rotation axis of the cable drum transversal to the movement direction of the piston in the cylinder, the windup point of the cable drum moves back and forth in longitudinal direction of the cable drum, thus transversal to the movement direction of the piston during windup, which is performed only in one layer for measuring cable sensors.
Since the piston is very close to the measuring cable sensor when the piston has moved proximal to the sensor, these are angular amounts which cannot be neglected anymore and which falsify the measuring results.
Additionally, there is in principle the risk that the cable jumps and that the cable drum is wound up in several layers, which also leads to a substantial falsification of the measuring result which is not detected from the outside, and the cable is then also highly loaded mechanically when being wound up at the sides of the cable drum.
In order to prevent this, complex additional measures for correct cable support are required.
However, when the measuring cable sensor is mounted with the rotation axis of the cable drum parallel to the movement direction of the piston, the measuring cable has to be deflected at least once from the unwinding direction into the movement direction of the piston, which leads to a significant reduction of the service life of the measuring cables, though the long but not unlimited service life of a measuring cable of a measuring cable sensor already constitutes a disadvantage in principle of installing measuring cable sensors at a position of this type, which is accessible only with difficulties, wherein the complexity of disassembling and replacing the measuring cable sensor in this position is extremely high.